To remove nitrogen compounds and carbon compounds from waste water in a sewage treatment facility, an activated sludge method employing decomposition and adsorption by microorganisms is used. To activate the microorganisms in the waste water, the microorganisms must be provided with oxygen. A diffuser apparatus is disposed in a bottom portion of an aeration tank in the sewage treatment facility or the like to feed oxygen into a mixed liquid containing activated sludge and waste water. A diffuser apparatus in which an aeration plate surface for releasing air bubbles is constituted by a membrane as shown in Japanese Patent Application Publication No. 2003-320388 and a diffuser apparatus in which the aeration plate surface is constituted by a thin metal plate as shown in Japanese Patent Application Publication No. 2006-61817 exist as diffuser apparatuses. The diffuser apparatus in which the aeration plate surface is constituted by a thin metal plate is superior in terms of durability and ease of maintenance inspections, and undergoes less pressure loss during a diffusing operation.
When a diffuser apparatus performs a diffusing operation continuously over a prolonged period, dirt components adhere to the interior and the surface of fine pores formed in the aeration plate surface, causing clogging. The main dirt component that causes this clogging is a biofilm formed by the microorganisms, which causes clogging by blocking the fine pores. The biofilm exerts a powerful adhesive force and cannot therefore be peeled away even when a gas pressure of the air or the like that is fed during the diffusing operation is increased.
The following method has been proposed as a method for removing a biofilm that causes clogging.
A method in which a biofilm produced by microorganisms adhered to the interior and surface of fine pores formed in a membrane is removed by expanding or contracting the membrane through an air blowing operation in which an amount of air blown by the diffuser apparatus is increased and decreased or the like has been disclosed as a method for use in a membrane-type diffuser apparatus by, for example, Japanese Patent Application Publication No. 2004-313938.
However, in a thin metal plate-type diffuser apparatus, the aeration plate surface is not flexible, and therefore a method such as that described above, which is applied to a membrane-type diffuser apparatus, cannot be used. Hence, at present, when microorganisms multiply and cause clogging in a thin metal plate-type diffuser apparatus, the thin aeration plate is cleaned by raising the diffuser apparatus to the water surface. As a result, maintenance of the diffuser apparatus involves large amounts of labor and expense.
Further, in both the membrane type and the thin metal plate type, a pore diameter of the fine pores formed in the aeration plate surface must be reduced to improve the oxygen transfer efficiency. However, clogging becomes steadily more likely to occur as the pore diameter decreases, and therefore an apparatus exhibiting a high oxygen transfer efficiency is more likely to become clogged. As described above, a diffuser apparatus that is likely to become clogged must be raised to the water surface frequently in order to clean the thin aeration plate, and therefore maintenance of the diffuser apparatus involves large amounts of labor and expense.